Forgeable high strength austenitic alloy with copper, molybdenum, and columbium-tantalum additions



United States Patent 6 i oRGEABLE HIGH STRENGTH AUSTENITIC ALLOY WITH COPPER, MOLYBDENUM, AND COLUMBIUM-TANTALUM ADDITIONS Fritz T. Eberle, Barberton, Ohio, assignor to The Babcock & Wilcox Company, New York, N. Y., a corporation of New Jersey Application July 30, 1954, Serial No. 446,876

4 Claims. (Cl. 75-125) This invention relates to forgeable alloy steels having enhanced stress rupture strength, corrosion resistance, and freedom from embrittlement in extended service, at elevated temperatures and stresses, and, more particularly, to a fully austenitic chrome-nickel-iron alloy steel attaining the foregoing properties with a minimum total alloy content.

For a number of years there has been a steady increase in the superheater outlet temperatures and pressures of vapor generators, with a resulting increase in the efiiciency and economy of turbines driving electric generators. These temperature and pressure increases required alloy steels of the columbium and titanium bearing 18Cr-8Ni AISI Types 347 and 321. With superheater outlet temperatures of 1050 F., the pressures involved are frequently substantially in excess of 2000 p. s. i.

With pressures of this order, the superheater tubing must have wall thicknesses of up to A3" for such 188 alloys to remain within their allowable working stresses. Such wall thicknesses are undesirable, not only from the standpoint of fabrication problems but also from the standpoints of heat transfer and thermal stress gradients across the Wall of the tubing. As a consequence, the increase in superheater outlet temperatures recently has been arrested at substantially the 1100 F. level.

Any further substantial increase in superheater outlet temperatures requires steel alloys capable of practical fabrication into tubing having wall thicknesses acceptable from the fabrication, heat transfer, and thermal stress gradient standpoints, and having long-time strength and corrosion resistance at temperatures in excess of 1350 F. and pressures substantially in excess of 2000 p. s. i. In addition, considering the large quantities of such tubing required in modern vapor generator installations, such alloys must have a low total alloy content in order to be economically feasible for use as superheater tubing.

There are known alloys which have long-time strength at high temperatures but which either have too high an alloy content to be economically practical for use as superheater tubing or are substantially non-forgeable, difiicult to force, or characterized by a loss of desirable properties in long-time service at elevated temperatures. The present invention is, accordingly, directed to a steel alloy capable of economically practical use as tubing operating at temperatures in excess of 1350" F. and pressures in excess of 2000 p. s. i., and having the lowest possible alloy content, being particularly low or lean in strategically important elements. The invention is par-" ticularly directed to such an alloy meeting the following requirements (1) Stress-rupture strength, at 1350 F., at least twice that of A181 Type 304 alloys, the most economical steel alloys commercially available for use at such elevated temperatures; v

(2) Adequateresistance to corrosion by superheated vapor and combustion gases at l350 F.;

(3) Adequate hot plasticity, for fabrication into tubing;

(4) Favorable mechanical properties;

2,824,795 Patented Feb. 25,1958

Percent Cr 15.0020.00 Ni 12.00-18.00 C 0.02- 0.15 Mn 0.25- 2.50 Si 0.10- 1.00

Balance iron with the usual impurities.

This base composition is a fully austenitic iron-chromenickel steel alloy of relatively low carbon and silicon content. The chromium content is sufiiciently high for adequate oxidation and corrosion resistance at temperatures. of the order of about 1500 F., and yet sufficiently low' to suppress sigma-phase formation. The nickel content; is sufiicient to maintain the alloy structure fully austenitic: over a wide range of variation in alloying additions. The: fully austenitic, or face-centered lattice, structure is im portant for maximum sustained high-temperature strength, the low carbon content assures hot plasticity and weldability, and the low silicon content is adequate insurance against micro-fissuring in welding.

The creep-rupture strength of this base composition is raised by suitable alloy addition designed to produce age hardening processes in the base composition by forming complex carbides or intermetallic compounds which are soluble in the base composition at very high temperatures. but insoluble or of limited solubility therein at lower tem-. peratures in the general vicinity of the contemplated use: temperature; i. e. of the order of 1350 F., or higher;

In accordance with the present invention, the creeprupture strength of the base composition is very substan tially increased by adding thereto Cb-Ta from 1.00% to 3.50%, Cu from 2.00% to 3.00%, and Mo from 1.00% to 2.00%. The invention alloy may be classed generallyas a l5Cr-l5Ni-2.5Cu-1.5Mo-2.5 Cb-T a steel alloy.

In the drawing, the single figure is a graphical comparison, at 1350 F., of the creep-rupture strength of the invention alloy and an-AISI Type 304 l8Cr-8Ni steel alloy.

In the invention alloy, the chromium content selected had to be high enough to insure adequate resistance to oxidation and scaling at a contemplated maximum use temperature of 1350 F. to 1450 F., and low enough to inhibit or minimize the formation of embrittling sigma phase. A chromium content of 15% to 17% is suitable for effecting these results. It is advisable to hold the chromium content on the low side since chromium, as well as most of the other elements available for strengthening the base composition, is a ferrite former promoting the weak, body-centered cubic lattice structure which has to be compensated by suitably increased additions of the relatively expensive, and strategically important, austenite forming nickel.

With a chromium content of 15% to 17%, a nickel content of 15% is 'sufiicient to neutralize the ferrite Similarly, if the nickel content in such an alloy is held at 15 an increase in the chromium content from 15% to 25% does not add materially to the creep strength,

The carbon content of the invention alloy is carefully} E'selec'ted 1 to assure higli combinations of. additives.

' forlsuperheater tubing, is as-follows:

V 1 Ci 7 '7 V eno strengthening the alloy-through formation 'of complex carbides yet not so high as to. affect adversely forgeability and lead to seams in tubing 'forme'd from the alloy. For this purpose, the ca'rbon 'content 'h'as' -a x imunt of 0.12% and preferablyis held between- -,03% and "0.05%.

'Manganese has a beneficial effect iip'onflhot working properties due to its -actionupon oiiygen' andFsulp hur,

; It is also desirable as' an ingredient'dueito its tendency to form 'austenite, although its potency, in this respect,

is inferior to -that of nickel, Hence,-5the preferred-mam ganesezcontent is 1.75%;which is near the upper 'end' of the range of manganese commonly found-in 18-8 type'alloys. a Silicon is a -strong-ferriteformer; and-should be kept at a'low value where it is .desired to-prornote'austenite formation. 7 the formation'of'strengthening Compounds, such-as silicides', with columbium and tantalum. It is also a powerful deoxidizer, andenhan'cesresistance .to oxidation, at high temperatures, by forming a tightly adherent protec tivescale, being: much more; efiective than ,chromiur n in this respect." By combining aJoW-rangechromiumcontent 'with a high-range silicon content, satisfactory, scaling resistance, with a minimum tendency to sigmaa hase embrittlement, is assured. For these reasons, a

silicon contentof substantially 0.75% is prefcrred. As stated, the base composition is strengthened by t alloy additions 'des'ignedi'toproduce age hardening processes. For this purpo se, the invention -alloy,'inclnding such addition, is solution heat treated at a high tempera- 3 ture, suchvas 2200F. to 2300 F., followed by an aging treatment, or by use, at'a lower temperature, such-as I 1350 F; With'suitable alloy additions,"a fine dispersion 35 a t Type 304 alloys.

of precipitated compounds in 'the lattice structure of the 1 matrix is achieved. .This fine dispersion resistsor-retar'ds ugh carbon content for i lhe preferred'percentages wrespectively; 2.50% 2.5 0%"and" 1:5 0%;

Alloys of substantially this preferred composition 4', J of Cb-Ta, Cu 1 and V have been compounded, solution heat-treated at2200 F.-2300 F; and aged at temperatures of the order, of 1300" F.'-1500 F. These 'alloys have then :been subjected to'stress rupture testsat 1350i F. for over.-7,000 hou'rsi: In the accompanying; drawing; :the stress 'r ipt'u're curve, plotted on a logarithmic scale with ruptliiistfength On the other,hand, .silicon participates in in'p. s'. i, as ordinates and hours 'undeFstressas abseissae, is given-with'actual values upto the'7;000?honrspoint; a

temperature and extr'apolated to 100,000 hoiirsithe test" being 13509 Curve-A represents the j stre of the invention alloys is substantially 19,300 'p. s. i. {well over twice that of the Type 304alloyl At l0,000,hours,--

the stress-rupture-strength'" of the inventionalloys is in found that, of; available strengthening-alloy additions, tantalum or columbium-tantalum, molybdenum, and copper,'in combination, are most potent in improving the rupture strength; at elevated temperatures, .of the-base denum is nearly as potent, in improving the high' te'm': perature rupture strength of the base composition, tantalum and columbium-tantalum. The copper, when Iused with'tantalum or colurnbium-tantalumiandmolyba 'denum, is: very eifectiveq in obtainingoptimum :stress* 7 t '45 composlt on. Pure'columb um, In thisrespect, is notjas effective as tantalum and columbium-tantalum. Molyb:

tion. .*There appears to be an optimum: concentration.

joreachelement which "app ears to'dif ferj for diiferent 1 Within the composition range 'PICVI 17.00% marin Ba'lauce'iron with thesiusual f s ously tabulated, a preferred composition of a forgeable, high-strength-at: "high-temperature alloy embodying the invention,;and

whichis lean inalloy content and economically practical 7 amen-2.00%;

excess of 14,300p. s. i. as comparedto ab out 4700'p. i,

for fliejTyP6 3 04. alloy, nearly three-'timesfas strong. At 100,000 hours, the indicated rupture strength ofgthe- -invention alloys is in excess'of10,600 p. s;ti,,-aboi1t fouri,

(4.) times'the 2700 p; s. i. value for 'the Type;304-alloy. Th'e'values' for the A181 Type 304 allo y: are takenz-from' ASTM-ASMES pec Tech Publ No. 124. a

i It will "be noted-that the, invention alloys havefstress a rupture strengths, at 1350" over'twice thatoft ej Iypei304 alloy. Consequently, they can be usedto form tubing suitablefor prolonged service at 'such temperatu're [and having reduced wall thicknesses acceptable 1 from the fabrication, thermal stress gradient, and econ-u 'omy'standpoints, while having a prolonged stress-re- 7 ing formed from-the sist'ancefatleast equal to that of tub L'The alloys embodying the titans withinthe following ranges:

, CID-T8. 1.85% 3B0} on 52.50 i 3:00; M 1 l 7 51.33 3.00 Balanc'e iron with the' usnalimpuritiesm "While a specific embodiment ofithe invention has aeni shown and described 'ing detailto illustrate-the appuca;

tion of the invention-principles, it will be-understood that=. 7 tion may be embodiedotherwisewithoutdethe inven 7 v parting-fronr.suchiprinciplest v QWha'tis c laimedis: i ".1; A- forgeable austenit under 7 stress at; temperatures:oii the- {o1 d e1 of 1300? 'F.; said-alloy having the followi'n composition: j

Mo are,

7 ss-Tuptureyalues:ofialloys I embodyingthe inventiomwhilecurye Brepresents those of an AISI Type-30418Gr+8Ni-alloy." It will be oh- ,7 served that, at 1,000 hours, the stress-rupture strengths;

a I a invention and; represented; ;by.-a curve A of the drawing-have percentage COllflPOSl-P Percent.

a if a N ic steel a1loy-having ;superior stressresistance and corrosion resistance properties, and: 'freedom 'f'ro rn impact embrittlemenhin extendedqservices exteiidedl'serviee 5 under stress at temperatures of the order of 1300 F.; said alloy having the following composition:

Cr 17.00% maximum. Ni 15.00% maximum. C 0.12% maximum. Mn 2.00% maximum. Si 0.75% maximum. Cb-Ta 1.50-3.50%. Cu 2.00-3.00%. Mo LOO-2.00%.

Balance iron with the usual impurities.

3. A forgeable austenitic steel alloy having superior stress resistance and corrosion resistance properties, and freedom from impact embrittlement, in extended service under stress at temperatures of the order of 1300 F.; said alloy having the following composition:

Cr 17.00% maximum. Ni 15.00% maximum. C 0.12% maximum. Mn 2.00% maximum. Si 0.75% maximum. Cb-Ta 2.5%. Cu 2.50%. M0 1.50%.

Balance iron with the usual impurities.

4. A forgeable austenitic steel alloy having superior stress resistance and corrosion resistance properties, and freedom from impact embrittlement, in extended service under stress at temperatures of the order of 1300 F.;

5 said alloy having the following composition:

Balance iron with the usual impurities.

References Cited in the file of this patent UNITED STATES PATENTS 20 2,540,509 Clarke Feb. 6, 1951 FOREIGN PATENTS 647,701 Great Britain Dec. 20, 1950 

1. A FORGEABLE AUSTENTIC STEEL ALLOY HAVING SUPERIOR STRESS RESISTANCE AND CORROSION RESISTANCE PROPERTIES, AND FREEDOM FROM IMPACT EMBRITTLEMENT, IN EXTENDED SERVICE UNDER STRESS AT TEMPERATURES OF THE ORDER OF 1300* F.; SAID ALLOY HAVING THE FOLLOWING COMPOSITION: PERCENT CR -- 15.00-20.00 NI -- 12.00-18.00 C -- 0.02- 0.15 MN -- 0.02- 0.15 SI -- 0.10-1.00 CB-TA -- 1.50-3.50 CU -- 2.00-3.00 MO -- 1.00-2.00 